Generally, in order to measure a flow rate by using a flowmeter, there is a demand for detecting a characteristic or state of the fluid and a state in a tube through which the fluid flows, together with the flow rate of a fluid to be measured. For example, in a manufacturing line for mixing a chemical solution or the like, the characteristic such as the conductivity or permittivity of the fluid is measured together with the flow rate of the fluid. Also, when a large amount of substance adheres to the inside of a measuring tube, the deposition state of the substance in the measuring tube is measured in order to know the maintenance cycle of the measuring tube. As for a sewer, a demand has arisen for measuring the state such as a level of the fluid and the deposition state of the substance adhering to the inside of the measuring tube together with the flow rate. Actually, the state of the fluid and the state in the measuring tube are measured by using a measuring device other than the flowmeter.
As described above, a demand has arisen for measuring the characteristic or state of the fluid or the state in the measuring tube though which the fluid flows, together with the flow rate of the fluid, and executing measurement processing by using basically the same hardware arrangement as that of a flowmeter. That is, there is a demand for selectively implementing various functions such as measurement of the flow rate of the fluid, measurement of the conductivity of the fluid, and simultaneous measurement of the flow rate and conductivity by using one measurement device. Since the flow rate and state are preferably measured at the same time, it is obviously important to measure the characteristic or state of the fluid regardless of the flow rate of the fluid.
When an electromagnetic flowmeter serves as the flowmeter, in addition to the above demands, a demand has arisen for measuring the characteristic or state of the fluid or the state in the measuring tube from the viewpoint of the self-diagnosis of the electromagnetic flowmeter. For example, when an insulator or the like adheres to the electrode, an electrode type flowmeter for extracting a potential from an electrode which is in contact with the fluid can neither accurately extract the potential, nor measure the flow rate with precision. To cope with this problem, when the resistance of the fluid containing the substance can be measured by using the same electrode, the deposition state of the substance adhering to the electrode can be measured, thus preventing any trouble that an abnormal flow rate measurement value is obtained. In a general electromagnetic flowmeter, an abnormal flow rate measurement value is obtained when the conductivity of the fluid falls outside a specific range. In this case, as long as the resistance of the fluid can be measured, it is determined that an output error which has occurred when the fluid having conductivity falling within the specific range flows originates from a change in the flow rate, or the fluid conductivity falling outside the specific range. As a result, the electromagnetic flowmeter can have a self-diagnosis function as the flowmeter.
As described above, a demand has arisen for meeting the request for executing various measurement processes in addition to the measurement process of the flow rate by using basically the same hardware arrangement as that of a flowmeter.
The solution to the request is not presented now. However, an electromagnetic flowmeter which detects a parameter other than the flow velocity is disclosed, as a relatively closer technique, in reference 1 (Japanese Patent Laid-Open No. 6-241855) and reference 2 (JNMIHF edition, “Flow Rate Measurement A to Z for Instrumentation Engineers”, Kogyo Gijutusha, 1995, pp. 147-148). In references 1 and 2, a device which measures the level, conductivity, and the like of the fluid is disclosed as an application of the electromagnetic flowmeter. Such electromagnetic flowmeter obtains the level of the fluid on the basis of the ratio between a signal electromotive force obtained from an electrode when driving exciting coils placed above and under a tube and a signal electromotive force obtained when driving the exciting coil placed above the tube by itself, and obtains the conductivity of the fluid on the basis of the ratio between the signal electromotive forces obtained when changing an input impedance of a pre-amplifier connected to the electrode.